There exist a number of methods for production of carbon nanostructures and carbon nanotubes. These may be divided in two main categories; high temperature methods and low temperature methods. Most of the high temperature methods are based on sublimation of carbon under an inert atmosphere, such as the electric arc discharge process, the laser ablation method and the solar technique. Low temperature methods are e.g. chemical vapour deposition (CVD) using the catalytic decomposition of hydrocarbons, gas phase catalytic growth from carbon monoxide, production by electrolysis, heat treatment of polymer, low temperature in situ pyrolyse or in situ catalysis. The main methods are further described below.
CVD (Chemical Vapour Deposition) is a method where carbon nanotubes are produced from gas phase by growing carbon nanotubes on a substrate by using large amounts of catalyst at a low temperature (600-1000° C.). The carbon nanotubes produced contains defects, resulting in bends on the structures. Also, the catalyst is present in the resulting carbon nanotubes in various amounts, from 50% and as low as 1-10%.
Arc methods are plasma methods where a DC electric arc discharge is established between an anode and a cathode only a few millimeters apart. This creates a rather small plasma arc in the area between the electrodes. Carbon evaporated from the carbon based (e.g. graphite) anode recondenses on the cathode in the form of a deposit containing carbon nanotubes. In the original arc method published by T. W. Ebbesen, Nature 358 (1992) no externally applied magnetic field generated by permanent magnets or electromagnets was used, and no recirculation of evaporated carbon could occur. This arc method is only suited for small-scale production of carbon nanotubes in arc reactors operating at very low current and power levels—typically a few kW. Upscaling to higher power levels by increasing the arc current and/or the electrode diameter seems not feasible because the deposited carbon nanotubes will be re-evaporated from the cathode.
In the conventional arc discharge method there is a point-to-point discharge between the electrodes. In order to improve this method, the cathode and the anode were shaped as a plate at the tip to create a plane-to-plane micro-discharge, and somewhat increase the plasma volume, published by Lee S. J. et al.: “Large scale synthesis of carbon nanotubes by plasma rotating arc discharge technique”, Diamond and Related Materials, 11, 2002, pages 914-917. Lee S. J. et al. correctly states: “Conventional arc discharge is a discontinuous and unstable process, and it can not produce the high quality of carbon nanotube in mass production. The nanotubes are produced on the cathode surface and the electrode spacing is not constant, so the current flow is not uniform and the electric fields are non-homogenous.” To overcome this the anode was mechanically rotated in order to create centrifugal forces so that the carbon vapour did not deposit on the cathode, but was transferred out of the plasma region and condensed on a collector at temperatures between 900-1100° C. This may enable continuous production, but still this method operates at very low power levels, in the range 1.6-3.6 kW, and is best suited for small-scale production. The mechanical rotation of the anode will not rotate the arc to any extent. However, electrode rotation is normally performed in order to obtain uniform electrode wear. Experimental results with a mechanical rotating anode is presented in Bae J. C. et al., “Diameter control of single-walled carbon nanotubes by plasma rotating electrode process”; Carbon, vol. 40, number 15, 2002, pages 2905-2911.
Further examples of electric arc methods are described in e.g. U.S. Pat. No. 5,277,038 and U.S. Pat. No. 6,451,175. In U.S. Pat. No. 6,451,175 the cathode is longitudinally vibrated to enhance the carbon nanotube deposition on the cathode.
Magnetic fields generated by permanent magnets or coils are introduced in other publications in order to overcome the stability problem. In particular the inventors of the European patent application EP1577261A (ANAZAWA, KAZUNORI) in claim 1 give detailed instructions on how to locate magnets and create a field stabilizing the arc. In addition, e.g. US patent application US20040084297, U.S. Pat. No. 6,902,655 (ANAZAWA et al) and JP08048510 (MIENO et al) “improve the efficiency” of the arc method, either by increasing the charged particle collision frequencies or by blowing the plasma away from the cathode. Again, upscaling to larger production units seems not feasible. Especially, EP1577261A mentions a drum/plate formed cathode which is rotated in order to enable scraping off the nanotubes continuously by a scraper at the cathode side, below the arc. This also provides even wear/abrasion of the electrodes. The magnetic field aims at stabilizing the arc between the anode and the cathode in order to avoid the arc to follow along the first part of the cathode rotation.
WO 2004/083119 describes a plasma method for continuous production of carbon based nanotubes, nanofibres and other nanostructures. Carbon precursor, catalyzer and carrier plasma gas are introduced in the reaction zone where the carbon precursor (preferably solid carbon particles) is vaporized. The hot plasma in the reaction zone is generated by arcs established by connecting an AC power source to two or three carbon electrodes. The gas-vapour mixture is then guided through a nozzle and into a quenching zone for nucleation. This apparatus bears a close resemblance to traditional plasma torches used for e.g. plasma spraying of refractory coatings, in which case evaporation of the particulate feedstock is not desired. The main problem with this method is that no recirculation of feedstocks and products from the quenching zone occurs.